869 research outputs found
Structure and properties of neutron stars in the Relativistic Mean - Field Theory
Properties of rotating neutron stars with the use of relativistic mean-field
theory are considered. The performed analysis of neutron star matter is based
on the nonlinear Lgrangian density. The presence of nonlinear interaction of
vector mesons modifies the density dependence of the rho field and influences
bulk parameters of neutron stars. The observed quasi-periodic X-ray
oscillations of low mass X-ray binaries can be used in order to constrain the
equation of state of neutron star matter. Having assumed that the maximum
frequency of the quasi periodic oscillations originates at the circular orbit
it is possible to estimate masses and radii of neutron stars.Comment: 18 pages, latex2e, 10 colour eps figures, submitted to International
Journal of Modern Physic
Gaps below strange star crusts
The gap caused by a strong electric field between the quark surface and
nuclear crust of a strange star is studied in an improved model including
gravity and pressure as well as electrostatic forces. The transition from gap
to crust is followed in detail. The properties of the gap are investigated for
a wide range of parameters assuming both color-flavor locked and non
color-flavor locked strange star cores. The maximally allowed crust density is
generally lower than that of neutron drip. Finite temperature is shown to
increase the gap width, but the effect is significant only at extreme
temperatures. Analytical approximations are derived and shown to provide useful
fits to the numerical results.Comment: 12 pages incl. 14 figures. To appear in Physical Review
Theoretical Models of Superbursts on Accreting Neutron Stars
We carry out a general-relativistic global linear stability analysis of the
amassed carbon fuel on the surface of an accreting neutron star to determine
the conditions under which superbursts occur. We reproduce the general
observational characteristics of superbursts, including burst fluences,
recurrence times, and the absence of superbursts on stars with accretion rates
below 10% of the Eddington limit. By comparing our results with observations,
we are able to set constraints on neutron star parameters such as the stellar
radius and neutrino cooling mechanism in the core. Specifically, we find that
accreting neutron stars with ordered crusts and highly efficient neutrino
emission in their cores (due to direct URCA or pionic reactions, for example)
produce extremely energetic (> 10^44 ergs) superbursts which are inconsistent
with observations, in agreement with previous investigations. Also, because of
pycnonuclear burning of carbon, they do not have superbursts in the range of
accretion rates at which superbursts are actually observed unless the crust is
very impure. Stars with less efficient neutrino emission (due to modified URCA
reactions, for example) produce bursts that agree better with observations.
Stars with highly inefficient neutrino emission in their cores produce bursts
that agree best with observations. All systems that accrete primarily hydrogen
and in which superbursts are observed show evidence of H- and He-burning
delayed mixed bursts. We speculate that delayed mixed bursts provide sufficient
amounts of carbon fuel for superbursts and are thus a prerequisite for having
superbursts. We compare our global stability analysis to approximate one-zone
criteria used by other authors and identify a particular set of approximations
that give accurate results for most choices of parameters. (abstract truncated)Comment: 43 pages, 18 figures, accepted by Ap
Comments on the paper ``Bare Quark Surfacees of Strange Stars and Electron-Positron Pair Emission''
In a recent paper (Ushov, PRL, 80, 230, 1998), it has been claimed that the
bare surface of a strange star can emit electron-positron pairs of luminosity
\~10^{51} ergs/s for about 10s. If true, obviously, this mechanism may explain
the origin of cosmic Gamma Ray Bursts. However, we point out that such a
mechanism is does not work because (i) if pair production really occurs the
supposed pre-existing supercritical electric field will be quenched and this
discharge process may at best release ~10^{24} ergs of electromagnetic energy,
and (ii) there is no way by which the trapped core thermal energy of few
10^{52} ergs can be transmitted electromagnetically on a time scale of ~10s or
even on a much larger time scale. The only way the hot core can cool on a time
scale of ~10 s or much shorter is by the well known process of emission of
nu-antinu pairs.Comment: Final version accepted in Phy. Rev. Lett. Main conclusion that the
mechanism by Usov does not work remains unchanged,
[email protected]
Maximum mass of a cold compact star
We calculate the maximum mass of the class of compact stars described by
Vaidya-Tikekar \cite{VT01} model. The model permits a simple method of
systematically fixing bounds on the maximum possible mass of cold compact stars
with a given value of radius or central density or surface density. The
relevant equations of state are also determined. Although simple, the model is
capable of describing the general features of the recently observed very
compact stars. For the calculation, no prior knowledge of the equation of state
(EOS) is required. This is in contrast to the earlier calculations for maximum
mass which were done by choosing first the relevant EOSs and using those to
solve the TOV equation with appropriate boundary conditions. The bounds
obtained by us are comparable and, in some cases, more restrictive than the
earlier results.Comment: 18 pages including 4 *.eps figures. Submitted for publicatio
Testing Approximations of Thermal Effects in Neutron Star Merger Simulations
We perform three-dimensional relativistic hydrodynamical calculations of
neutron star mergers to assess the reliability of an approximate treatment of
thermal effects in such simulations by combining an ideal-gas component with
zero-temperature, micro-physical equations of state. To this end we compare the
results of simulations that make this approximation to the outcome of models
with a consistent treatment of thermal effects in the equation of state. In
particular we focus on the implications for observable consequences of merger
events like the gravitational-wave signal. It is found that the characteristic
gravitational-wave oscillation frequencies of the post-merger remnant differ by
about 50 to 250 Hz (corresponding to frequency shifts of 2 to 8 per cent)
depending on the equation of state and the choice of the characteristic index
of the ideal-gas component. In addition, the delay time to black hole collapse
of the merger remnant as well as the amount of matter remaining outside the
black hole after its formation are sensitive to the description of thermal
effects.Comment: 10 pages, 6 figures, 9 eps files; revised with minor additions due to
referee comments; accepted by Phys.Rev.
Analytical models for quark stars
We find two new classes of exact solutions to the Einstein-Maxwell system of
equations. The matter content satisfies a linear equation of state consistent
with quark matter; a particular form of one of the gravitational potentials is
specified to generate solutions. The exact solutions can be written in terms of
elementary functions, and these can be related to quark matter in the presence
of an electromagnetic field. The first class of solutions generalises the Mak
and Harko model. The second class of solutions does not admit any singularities
in the matter and gravitational potentials at the centre.Comment: 10 pages, To appear in Int. J. Mod. Phys.
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